1. Field of the Invention
This invention relates to pumps utilized to circulate coolant water in nuclear reactors. More particularly, it relates to a thermal barrier which protects pump seals and bearings from the hot reactor coolant water and to a pump incorporating such a thermal barrier.
2. Background Information
The pumps which circulate coolant water through a nuclear reactor are subjected to harsh conditions. The reactor coolant water in a pressurized water reactor (PWR) is typically at a pressure of about 2,250 psi and a temperature in excess of 500 degrees Fahrenheit. The bearings and seals for the pump shaft are protected from these conditions by a thermal barrier. A common type of thermal barrier includes a cylindrical cover which seats in a recess in the pump housing where the pump shaft extends into the pump chamber. This cover has an end wall through which the pump shaft extends into the pump chamber. Coolant water is injected through a flange on the opposite end of the cover seated in the pump housing and flows outward into the pump chamber through a clearance between the pump shaft and the opening in the end wall of the cover. As a back-up to the cooling provided by this injected water, a stack of pancake cooling coils encircle the shaft under the cover. Inlet and outlet sections of the pancake cooling coils extend axially from the periphery of the coil stack and through the cover flange. A separate supply of cooling water can be circulated through this closed loop system. Additional thermal protection is provided by an annular insulator disposed against the inner surface of the cover sidewall. Such thermal barriers maintain the temperature of the water inside the cover well below the 550.degree. Fahrenheit of the reactor coolant water being pumped and also below the 220.degree. Fahrenheit maximum temperature for the seals and bearings.
However, after many years of service some cracks have developed at the intersection between the end and side walls of the cover, in the weld between the cover side wall and flange, and in the flange adjacent to the pancake cooling coil inlet and outlet penetrations and the penetration for the injection water.
There is a need therefore for an improved thermal barrier for reactor coolant pumps and a reactor coolant pump incorporating such an improved thermal barrier.